1. Technical Field
The present invention relates in general to the field of electronics, and in particular to electronic chips that generate extraneous heat during normal operation. More particularly, the present invention relates to a method and system for conducting heat away from an integrated circuit, which still more particularly may be a microprocessor.
2. Description of the Related Art
In a typical personal computer (PC), the main heat-generating component among the logic circuits is the processor, also referred to as the Central Processing Unit (CPU) or microprocessor (MP). As illustrated in FIG. 1, a processor 102 is mounted in a socket 104, which is mounted on a (printed) circuit board 106 by mating pins 108 from the processor 102 into the socket 104. As processors continue to grow in performance, so does the heat generated by the processors. To remove heat from processor 102, a heat sink (HS) 110, having a HS base 112 and a plurality of fins 114, is secured to processor 102 by a strap 116. Heat is conducted from the processor 102 to the HS base 112 and the fins 114, which dissipate heat by conduction and convection to ambient air surrounding fins 114.
There are two main thermal resistances to heat that is to be dissipated away from processor 102. The first of these two resistances is caused by the interface between processor  102 and HS base 112, and is referred to as “R Case to HS,” which describes the heat transfer resistance between the case of the processor 102 and the HS 110. The second resistance, known as “R HS to air,” is the internal heat transfer resistance of the HS 110 itself, including the material resistance of HS base 112 and fins 114 as well as the heat transfer resistance of the interface between HS 110 and ambient air, especially the air proximate to fins 114.
The temperature differential between processor 102 and an ambient environment, such as air, is called ΔT. For example, if the operating temperature of processor 102 is 75° C., and the ambient temperature around heat sink 110 is 35° C., then ΔT=75° C.−35° C.=40° C.
Heat resistance is properly the inverse of thermal conductivity, which is usually defined as watts per meter-Kelvin, thus resulting in thermal resistance as being meters-Kelvin per watt. However, by convention, heat resistance in electronics is typically defined as ΔT per watt of power generated by the electronic device. Expressed as a formula, then, where ΔT is the difference in the temperature (in Celsius) between the processor and the ambient air, P is the wattage of the processor, and R is the thermal resistance to heat being transferred away from the processor, then:
  R  =            Δ      ⁢                          ⁢      T        P  with R generally expressed in units of “degrees C/W” (temperature difference in degrees Celsius per Watt of energy).
In modern computers, the interface resistance between processor 102 and the bottom of HS base 110 (“R Case to HS”) accounts for over half of the total heat transfer resistance. Since air is a very poor conductor of heat, the most effective type of heat transfer from processor 102 to HS base 112 is by heat conduction via contacting surfaces of the bottom of HS base 112 and the top of processor 102. However, minor warping, pits and other features of both these surfaces result in only 1% to 5% of the surfaces actually being in contact. To address this lack of direct physical contact, several approaches have been taken in the past. One approach is to lap and polish the surfaces, but this is time consuming and usually cost prohibitive. Another approach is to use a contact interface, such as a grease 118, which is usually a thermally conducting silicon or filled hydrocarbon grease that conducts heat from processor 102 to HS 110. However, grease 118 is messy and difficult to replace in the field, and fillings, such as metals, used to increase thermal conduction are expensive. Other materials have been suggested to replace grease 118, including graphite material such as Union Carbide's GRAFOIL™, but with only limited improvement over the use of grease 118.
What is needed therefore, is a device that reduces interface thermal resistance between two imperfectly flat surfaces by promoting direct physical contact between the two surfaces, such as the top of processor 102 and the bottom of HS base 112.